It is known to prepare sodium fluoride by reacting sodium carbonate or sodium hydroxide with hydrofluoric acid. In this reaction, separating off the solid NaF from the mother liquor comprising from 4 to 5% by weight NaF in dissolved form can be improved by adding alcohol (Ullmann's Encyclopedia of Ind. Chem., 5th Ed., 1997, Wiley VCH, Weinheim). According to Remy, Lehrbuch der Anorg. Chem. [Textbook of Inorganic Chemistry] volume 1, 11th Edition, Leipzig, 1960, p. 933, on the introduction of hydrogen fluoride into dilute sodium hydroxide solution or sodium carbonate solution, NaF precipitates out as a white precipitate.
The industrial preparation is preferably performed from the less costly raw materials cryolite (Remy) or hexafluorosilicic acid (JP-A 71-03253, DD-A 220 587).
For the production of pure sodium fluoride, as is required, for example, for the preparation of pharmaceutical compositions and toothpaste, and for adding to drinking water, pure sodium hydroxide solution and hydrogen fluoride purified by distillation, which can be used in anhydrous form or as aqueous hydrofluoric acid, are suitable in particular. In this process, the use of 50% strength sodium hydroxide solution and hydrogen fluoride offers the advantage that, owing to the heat of reaction, virtually all the water evaporates and thus no costs arise for working up the mother liquor. However, the industrial implementation of the process is highly problematic because of the corrosivity of the reactants at the high process temperature and the high solids content in the reaction mixture. In addition, sodium hydroxide solution which, in contrast to the traditional amalgam process, in which it arises directly as a 50% strength lye, in the modern membrane process, in which it arises as a 30 to 32% strength by weight lye, must be concentrated with relatively high expenditure to 50% by weight NaOH content. Likewise, hydrofluoric acid arises in various processes in high purity as a dilute acid, which is suitable in principle for NaF preparation.
The use of less costly sodium hydroxide solution of low concentration or aqueous hydrofluoric acid is opposed by the fact that, in their reaction, relatively large amounts of mother liquor containing 4 to 5% by weight of dissolved NaF arise, the partial precipitation of which by adding alcohol would not simplify the problems of disposal.
The object of the present invention is to provide a process which is expedient from economic and ecological aspects for preparing sodium fluoride, which process, in particular, also permits the use of the raw materials diluted to a greater or lesser extent.